The present invention relates generally to refractory compositions useful in metallurgical applications and, more particularly, to refractory materials which are resistant to the erosive and corrosive effects of molten steel treated with calcium.
Heretofore, it has been common practice to employ alumina graphite refractory compositions in the sliding gate valves which control the flow of molten steel from a ladle to a tundish and from the tundish to a continuous casting mold or molds. Sliding gate valves are well-known in the art as exemplified by U.S. Pat. No. 4,415,103 to Shapland et al.
Development of more aggressive grades of steel resulting from special alloy additions or chemical ladle treatments, in particular, calcium deoxidation practices, have caused markedly increased chemical attack of the refractory components in the slide gate valve contacting the molten metal. In order to resist such erosive and corrosive attack, it has been proposed to use oxide bonded zirconia material. In addition, a zirconia carbon material is disclosed in U.S. Pat. No. 4,917,276 to Shikano et al. The '276 patent teaches a refractory composition for a sliding gate nozzle formed of a zirconia base refractory material composed of more than 53% by weight of partially stabilized zirconia having less than 10 mesh grain size and up to 30% by weight unstabilized zirconia. The material also contains 1- 7% by weight of metallic silicon powder having less than 100 mesh grain size, and 3 to 10% by weight carbon powder having less than 100 mesh grain size. The '276 patent further discloses that the zirconia base material should contain no alumina or silica but fails to attach any significance to the type of stabilized zirconia to be employed.
Typically, a sliding surface for a refractory plate should possess a hot strength up to 800 psi (56 Kg/cm.sup.2) and a cold strength up to 2000 psi (140 Kg/cm.sup.2) in order to maintain the necessary sliding integrity during service. It has been found that commonly used lime or calcia stabilized zirconia, while having outstanding cold strength properties, exhibits a dramatic drop in hot strength physical properties. Lime stabilized zirconia graphite material exhibits hot strength properties on the order of 150 to 400 psi which is not suitable for long term service as a slide gate plate. It is theorized that the impurities present in the lime stabilized zirconia migrate to the grain boundaries where they react to form a low temperature glassy phase which is incapable of resisting the higher temperatures.
The present invention overcomes the problems encountered in the prior art and provides a refractory composition for use in slide gate plates and inserts therefore which exhibit outstanding erosion and corrosion resistance to chemically aggressive steels while also possessing superior hot and cold physical properties.